There are tens of papers on this subject, maybe more. For various applications and technologies or specification. Your post too general.
Any way you could pick the Index of The Transaction of IEEE-MTT and you will find many references, there are books with whole chapters on this subject.(MYJ aERTECH hOUSE, Ian Hunter-Theory and Design of Microwave Filters.
There are examples of diplexer design that come with "serenede 8.71", also with "Eagleware". There is a wealth of information on this subject.